Sounds like the leeward boat had his thinking cap on while the other two were dozing. Nice going. The most windward boat should either have overrun the mark or tacked. With his lack of steerage he'd have ended up head to wind but that's just his tough luck.
If you're trying to park on the line you need to try and protect your leeward lane out of there. I've thought a few times about leaving my tiller extension sticking out to leeward for precisely this reason, I can't see anything in the rules that would prevent it, and because of it's length it should buy me a clear half boat's width or more. I've seen some 49ers benefit from having their double tillers because the leeward tiller sticks out beyond the leeward rack.